Receiving a plurality of radio frequency bands

ABSTRACT

A radio frequency receiver comprises a plurality of parallel receiving paths, wherein each path can receive a radio frequency signal in one of a plurality of radio frequency bands and amplify the received signal in a low noise amplifier. The amplified signals from the plurality of parallel paths are combined to one combined radio frequency signal in a common summation node and down-converted to a lower frequency signal in a mixer circuit. Each low noise amplifier comprises a low noise transconductance circuit providing a current signal to drive the common summation node, and an automatic gain control circuit in each path compensates for variations in signal strength independently of signal strengths of signals received by the other receiving paths. The receiver is suitable for simultaneous multiple band reception, where received signal strength can vary between the frequency bands.

TECHNICAL FIELD

The invention relates to a radio frequency receiver configured toreceive a plurality of radio frequency bands, a wireless communicationsdevice comprising at least one such radio frequency receiver, and amethod of receiving a plurality of radio frequency bands.

BACKGROUND

Wireless communication technologies continue to evolve to meet thedemand for increased data throughput. This is addressed on many levelswith different approaches including higher order modulation,multiple-input and multiple-output (MIMO), scheduling, increasedbandwidth, and so on. One way of meeting the challenging capacity gainsin next generation wireless networks (e.g. 5G) is by supporting largecommunication bandwidth. The frequency utilization is likely to spanseveral bands, often denoted communication on non-contiguous intra-bandas well as inter-band. Such trend in the telecommunication systems canbe solved by multiple parallel receiving paths and is the most commonimplementation in the literature to date.

One common solution to combine multiple radio frequency bands in awideband signal is to use a multiband receiver supporting simultaneouslytwo or more frequency bands. The most straightforward way of doing thatis to implement the multiband receiver with an entire radio frequencyreceiver for each radio frequency band. Each of these parallel receiversis then designed to receive a specific radio frequency band. This is themost common solution of the multiband support, where few modificationsare needed on top of a copy paste action on an available singlefrequency band receiver.

Although this solution can be implemented by just using a number ofwell-known single frequency band receivers in parallel, it also hasseveral disadvantages.

First, it is area consuming as baseband (or low-IF) blocks arereplicated. For example, filtering capacitors become large whenoperating on low bandwidth signals. For the same reason, it is alsoquite power consuming since multiple base band stages are needed.

Of further disadvantages, it can be mentioned that there is a need forunique and separate local oscillator signal generation for eachfrequency translating mixer. The local oscillator generation willconsume considerable power, and more importantly, unavoidableinteraction between these different local oscillator frequencies cancause system performance degradation.

Each mixer should be a quadrature mixer, which will generate imbalancebetween I and Q path. Any such I/Q imbalance may e.g. cause the image ofa first signal in a first band to at least in part overlap a second(desired) signal in the first band or in a second band and thus inhibitor reduce the ability to detect second signal. I/Q-imbalance requirementmay call for calibration, which is then needed for each receiver branch.Such estimation and compensation procedure can only be achieved with thewanted accuracy in the digital base band. Each mixer should have its ownI/Q imbalance calibration, which is very hard to implement, especiallyif it becomes frequency dependent and the number of parallel branchesincreases.

An alternative solution that introduces the idea of reusing some of thereceiver blocks is the combination of the narrower signals after thedown converting stage, i.e. at base band or low intermediate frequencyband. This implementation allows reducing the impact of the multiplerouting since a virtual ground is available already at base band, if adirect conversion receiver using a well-established current passivemixer topology is chosen. Since the baseband (or low-IF) blocks are nolonger replicated, this solution is less area consuming, and for thesame reason, it is also less power consuming. However, the otherdisadvantages mentioned above are still present also in this solution.

In some solutions, the signals may be added at radio frequency in power,using power combiners. However, also combining the signals in power atradio frequency has some drawbacks. Summation in power will introduceloss in the combiner when supporting isolation between the ports andsupporting a rich span of frequencies, i.e. frequency selective powercombiners are not considered applicable for this scenario. Further, thedriving impedance shown to the mixer, especially if the mixer is apassive one, can be too low, thus degrading the performance of thedown-conversion.

US 2013/043946 describes a wireless device including multiple receiversto support different frequency bands, thus trying to reduce circuitryand cost. This solution includes a plurality of low noise amplifiershaving outputs that are combined before the combined radio frequencysignal is down-converted in a common mixer. Although this disclosurefurther reduces the consumption of circuit area and power, it does notdisclose any possibility to compensate for different signal levels inthe different frequency bands, and since received signal strength canvary considerably between the frequency bands, the suggested solution isnot suitable for simultaneous multiple band reception. Further, theimpedance level of the combined low noise amplifiers outputs will dependon the number of active low noise amplifiers, and as a consequence alsothe frequency response and the linearity of the circuit will vary independence of the number of received bands.

SUMMARY

Therefore, it is an object of embodiments of the invention to provide aradio frequency receiver capable of receiving a plurality of radiofrequency bands, and which is suitable for simultaneous multiple bandreception, where received signal strength can vary between the frequencybands.

According to embodiments of the invention the object is achieved in aradio frequency receiver configured to receive a plurality of radiofrequency bands, the radio frequency receiver comprising a plurality ofparallel receiving paths, wherein each receiving path is configured toreceive a radio frequency signal in one of said plurality of radiofrequency bands and to amplify the received radio frequency signal in alow noise amplifier, the receiver being configured to combine amplifiedradio frequency signals from the plurality of parallel receiving pathsto one combined radio frequency signal in a common summation node and todown-convert said combined radio frequency signal to a lower frequencysignal in a mixer circuit. The object is achieved when each one of saidlow noise amplifiers comprises a low noise transconductance circuitconfigured to provide a current signal to drive said common summationnode, and an automatic gain control circuit configured to compensate forvariations in signal strength of the radio frequency signal received bythe receiving path comprising the low noise amplifier independently ofsignal strengths of radio frequency signals received by other receivingpaths of the radio frequency receiver.

When the receiver is configured to perform an automatic gain controlfunction separately in each receiver path, simultaneous multiple bandreception can be provided, also where received signal strength variesconsiderably between the received frequency bands.

In some embodiments, the radio frequency receiver further comprises acurrent driver circuit having an input connected to said commonsummation node and an output connected to an input of said mixercircuit. In this way, it is possible to keep the impedance at thesummation node low, making the solution intrinsically wider band andallowing for an increased number of radio frequency branches. The lowimpedance summation node also reduces the voltage swing due to themultiple signals, so that the overall linearity is not degraded. Thecurrent driver also provides isolation between the radio frequencysumming node and the radio frequency input of the mixer, thereby keepingthe loading impedance driving the mixer independent of the number ofinput stages.

The current driver circuit may comprise circuitry for adjusting itsinput impedance in dependence of a number of parallel receiving pathsreceiving radio frequency signals at a given time. The possibility ofincreasing or decreasing the impedance of the current driver dynamicallywhen the number of active receiving paths decreases or increases allowsthe frequency response and the linearity performance to be keptconstant, so that the frequency and linearity performance does notdepend on the number of radio frequency inputs enabled.

The current driver circuit may further comprise an auxiliary outputconnected to circuitry for measurement and correction of I/Q imbalance,said auxiliary output being isolated from the output connected to aninput of said mixer circuit. Having a separate I/Q measurement outputisolated from the receive path ensures that I/Q imbalance estimation andcompensation can be performed independently of the number of activeradio frequency bands and without affecting the receive path.

The radio frequency receiver may further be configured to utilize saidauxiliary output for at least one of power detection, 1/f noise removaland transmission signal cancellation.

In some embodiments, the mixer circuit is configured to down-convertsaid combined radio frequency signal to a baseband signal.

A wireless communications device may comprise at least one radiofrequency receiver as described above. In this way, the wirelesscommunications device benefits from the described advantages of theradio frequency receiver. In one embodiment, the wireless communicationsdevice is a base station for a wireless communications system. Inanother embodiment, the wireless communications device is a mobile phoneuse in a wireless communications system.

As mentioned, the invention further relates to a method of receiving aplurality of radio frequency bands in a radio frequency receivercomprising a plurality of parallel receiving paths, the methodcomprising the steps of receiving in each receiving path a radiofrequency signal in one of said plurality of radio frequency bands;compensating, in an automatic gain control circuit in each one of saidlow noise amplifiers, for variations in signal strength of the radiofrequency signal received by the receiving path comprising the low noiseamplifier independently of signal strengths of radio frequency signalsreceived by other receiving paths of the radio frequency receiver;amplifying in each receiving path the compensated radio frequency signalin a low noise amplifier; providing by a low noise transconductancecircuit in each one of said low noise amplifiers a current signal todrive a common summation node; combining amplified and compensated radiofrequency current signals from the plurality of parallel receiving pathsto one combined radio frequency signal in said common summation node;and down-converting said combined radio frequency signal to a lowerfrequency signal in a mixer circuit.

When variations in signal strength is compensated by performing anautomatic gain control function separately in each receiver path,simultaneous multiple band reception can be provided, also wherereceived signal strength varies considerably between the receivedfrequency bands.

In some embodiments, the method further comprises the step of providingthe combined radio frequency signal from said common summation node toan input of said mixer circuit by a current driver circuit having aninput connected to said common summation node and an output connected toan input of said mixer circuit. In this way, it is possible to keep theimpedance at the summation node low, making the solution intrinsicallywider band and allowing for an increased number of radio frequencybranches. The low impedance summation node also reduces the voltageswing due to the multiple signals, so that the overall linearity is notdegraded. The current driver also provides isolation between the radiofrequency summing node and the radio frequency input of the mixer,thereby keeping the loading impedance driving the mixer independent ofthe number of input stages.

The method may further comprise the step of adjusting an input impedanceof the current driver circuit in dependence of a number of parallelreceiving paths receiving radio frequency signals at a given time. Thepossibility of increasing or decreasing the impedance of the currentdriver dynamically when the number of active receiving paths decreasesor increases allows the frequency response and the linearity performanceto be kept constant, so that the frequency and linearity performancedoes not depend on the number of radio frequency inputs enabled.

The method may further comprise the step of providing by the currentdriver circuit an auxiliary output, isolated from the output connectedto an input of said mixer circuit, to circuitry for measurement andcorrection of I/Q imbalance. Having a separate I/Q measurement outputisolated from the receive path ensures that I/Q imbalance estimation andcompensation can be performed independently of the number of activeradio frequency bands and without affecting the receive path.

The method may further comprise the step of further utilizing saidauxiliary output for at least one of power detection, 1/f noise removaland transmission signal cancellation.

In some embodiments, the method may further comprise the step ofdown-converting said combined radio frequency signal in said mixercircuit to a baseband signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described more fully below withreference to the drawings, in which

FIG. 1 shows a block diagram of a radio frequency receiver for a singleradio frequency band;

FIG. 2 shows a multiband receiver with an entire radio frequencyreceiver for each radio frequency band;

FIG. 3 shows a multiband receiver where signals are combined after downconverting to baseband or intermediate frequency;

FIG. 4 shows a multiband receiver where radio frequency signals arecombined before down converting to baseband or intermediate frequency;

FIG. 5 shows a multiband receiver where radio frequency signals areamplified in a low noise transconductance in each path and combinedbefore down converting to baseband or intermediate frequency;

FIG. 6 shows a block diagram of an example of an implementation of a lownoise transconductance of FIG. 5;

FIG. 7 shows an example of an implementation on transistor level of alow noise transconductance of FIG. 5;

FIG. 8 shows a multiband receiver as in FIG. 5 where the combined signalis coupled to a down-converting mixer through a current driver;

FIG. 9 shows an example of an implementation of a current driver of FIG.8;

FIG. 10 shows a base station and a mobile station in which a multibandreceiver can be used; and

FIG. 11 shows a flow chart illustrating a method of receiving aplurality of radio frequency bands in a radio frequency receiver.

DETAILED DESCRIPTION

FIG. 1 illustrates an example of a block diagram of a radio frequencyreceiver 1 for a specific radio frequency band. The radio frequencyreceiver 1 can be used in e.g. a wireless communications device, such asa base station or a mobile phone for use in a wireless communicationssystem. In this example, a radio frequency signal received by an antenna2 is connected to an input band pass filter 3 having a pass bandcorresponding to the specific radio frequency band of the receiver. Theband pass filtered signal is then amplified in a low noise amplifier 4and fed to a quadrature mixer 5, in which it is down converted to abaseband quadrature input signal comprising I-phase and Q-phasecomponents or to a corresponding intermediate frequency signal. In thisexample, the down-converted signal is then fed via a filter 6 and anamplifier 7 to an analog-to-digital converter 8 for further processing.

In wireless communication technologies there is an increasing demand forincreased data throughput. One way of meeting the challenging capacitygains in next generation wireless networks (e.g. 5G) is by supportinglarge communication bandwidth. The frequency utilization is likely tospan several bands, often denoted communication on non-contiguousintra-band as well as inter-band.

This trend in the telecommunication systems can be solved by multipleparallel receiving paths. Thus multiple radio frequency bands in awideband signal can be combined by using a multiband receiver supportingsimultaneously two or more frequency bands.

FIG. 2 shows a straightforward way of doing that by implementing themultiband receiver with an entire radio frequency receiver for eachradio frequency band. The receiver 10 has a number of parallel receiverpaths 1.1, 1.2, . . . , 1.n, each of which like the receiver 1 in FIG. 1is connected to the antenna 2 and comprises an input band pass filter 3,a low noise amplifier 4, a quadrature mixer 5, a filter 6, an amplifier7 and an analog-to-digital converter 8. Each of these parallel receiversis designed to receive a specific radio frequency band, e.g. bydesigning the input band pass filter 3 to have a pass band correspondingto the specific radio frequency band of that receiver.

This is the most common solution of the multiband support, where fewmodifications are needed in addition to a copy paste action on theavailable single frequency band receiver. Although this solution can beimplemented by just using a number of well-known single frequency bandreceivers in parallel, it also has several disadvantages.

The solution is area consuming as the baseband (or low-IF) blocks, i.e.the filter 6, the amplifier 7 and the analog-to-digital converter 8, arereplicated. For example, filtering capacitors become large whenoperating on low bandwidth signals. For the same reason, the receiver isalso quite power consuming since multiple base band stages are needed.Further, there is a need for unique and separate local oscillator signalgeneration for each frequency translating mixer 5. The local oscillatorgeneration will consume considerable power, and more importantly,unavoidable interaction between these different local oscillatorfrequencies can cause system performance degradation.

Each mixer 5 should be a quadrature mixer, which will generate imbalancebetween I and Q path. Any such I/Q imbalance may e.g. cause the image ofa first signal in a first band to at least in part overlap a second(desired) signal in the first band or in a second band and thus inhibitor reduce the ability to detect the second signal. I/Q-imbalancerequirement may call for calibration, which is then needed for eachreceiver branch. Such estimation and compensation procedure can only beachieved with the wanted accuracy in the digital base band. Each mixershould have its own I/Q imbalance calibration, which is very hard toimplement, especially if it becomes frequency dependent and the numberof parallel branches increases.

FIG. 3 shows an alternative solution that introduces the idea of reusingsome of the receiver blocks by combining the narrower signals after thedown converting stage, i.e. at base band or low intermediate frequencyband. Like the receiver 10 of FIG. 2, the receiver 20 in FIG. 3 has anumber of parallel receiver paths 11.1, 11.2, . . . , 11.n, each ofwhich is connected to the antenna 2. Each path comprises an input bandpass filter 3, a low noise amplifier 4 and a quadrature mixer 5, whilethe filter 6, the amplifier 7 and the analog-to-digital converter 8 areshared by all paths. Each of the parallel receiver paths is also heredesigned to receive a specific radio frequency band, e.g. by designingthe input band pass filter 3 to have a pass band corresponding to thespecific radio frequency band of that receiver.

This implementation allows reducing the impact of the multiple routingsince a virtual ground is available already at base band, if a directconversion receiver using a well-established current passive mixertopology is chosen. Since the baseband (or low-IF) blocks are no longerreplicated, this solution is less area consuming, and for the samereason, it is also less power consuming. However, the otherdisadvantages mentioned above are still present also in this solution.

FIG. 4 shows a receiver, where the narrower signals from the multiplereceiver paths are combined before the down-converting stage in themixer 5. Also the receiver 30 in FIG. 4 has a number of parallelreceiver paths 21.1, 21.2, . . . , 21.n, each of which is connected tothe antenna 2. Each path comprises an input band pass filter 3 and a lownoise amplifier 4, while the quadrature mixer 5, the filter 6, theamplifier 7 and the analog-to-digital converter 8 are shared by allpaths. Each of the parallel receiver paths is also here designed toreceive a specific radio frequency band, e.g. by designing the inputband pass filter 3 to have a pass band corresponding to the specificradio frequency band of that receiver.

In an embodiment of a receiver, shown as the receiver 40 in FIG. 5 anddescribed below, the low noise amplifier 4 in each receiver path of thereceiver 30 in FIG. 4 has been replaced by a low noise transconductance14. Thus, also the receiver 40 in FIG. 5 has a number of parallelreceiver paths 31.1, 31.2, . . . , 31.n, each of which is connected tothe antenna 2. Each path comprises an input band pass filter 3 and a lownoise transconductance 14, while the quadrature mixer 5, the filter 6,the amplifier 7 and the analog-to-digital converter 8 are shared by allpaths.

This receiver 40 implements a solution for receiving simultaneouslymultiple radio frequency bands to be combined into a wideband signal.The radio frequency bands can be either contiguous or non-contiguous.The apparatus is divided in multiple radio frequency path blocks addedtogether at radio frequency. In addition to just amplification, the lownoise transconductance 14 can include different functions, such asfilters, Automatic Gain Control (AGC), Variable Gain Amplifier (VGA),Programmable Gain Amplifier (PGA) and analog signal processing. Thedifferent functions of the individual low noise transconductances 14 areindependent of the corresponding functions in the other receiver paths,thus allowing reception of multiple bands with different signalstrength. Without separated AGC functionality for each radio frequencybranch, the solution would not be suitable for a simultaneous multipleband receiver.

The receiver 40 allows reception of multiple radio frequency bandswithout introducing any additional source of I/Q imbalance compared tothat of a conventional single band receiver. This is due to the factthat there is always only one mixer, since the number of down-convertingstages is not correlated to the number of radio frequency branches or,in other words, to the number of radio frequency bands received.

FIG. 6 shows a block diagram of an example of an implementation of thelow noise transconductance 14. Although the low noise transconductance14 in FIG. 5 is shown as being single-ended, the implementation in FIG.6 is shown as differential, which will typically be the case. Thedifferential input signals in+ and in− are fed to an AGC circuit 19performing the AGC functionality for the low noise transconductance 14in this receiver path. The AGC circuit 19 comprises a step attenuator 16for each differential input signal in+ and in− and an AGC control unit17 that controls the step attenuators 16 based on signal strengthsdetected at the inputs, so that the signals can be attenuatedaccordingly in the step attenuators 16. The step attenuators 16 can be ageneric design and/or an outside stand-alone component. The level ofattenuation is provided according to the detection of the signals cominginto the different inputs. Each input can have different attenuationlevels. The differential signals attenuated in the step attenuators 16are then fed to a transconductor circuit 18 providing the differentialoutput signals out+ and out− in the current domain.

FIG. 7 is a more detailed illustration on transistor level, according toan embodiment, of parts of the circuit in FIG. 6. For reasons ofsimplicity, the AGC control unit 17 is not shown in FIG. 7. Also theimplementation in FIG. 7 is shown as differential, which will typicallybe the case. The differential input signal in+ and in− is firstattenuated in the step attenuators 16 performing the AGC functionalitytogether with the AGC control unit 17 of FIG. 6 for the low noisetransconductance 14 in this receiver path. The level of attenuation isprovided according to the detection of the signals coming into thedifferent inputs. Each input can have different attenuation levels. Inthe transconductor circuit 18, the transistors M₁ and M₂ in each half ofthe circuit provide input matching and amplification. The attenuatedsignals from the step attenuators 16 are coupled to transistors M₂through an AC coupling capacitor C and connected to ground through aninductance L. A generic load Z is shown, but it can also be a morecomplex bias network.

With the separated AGC functionality for each radio frequency branch,the receiver 40 of FIG. 5 is more suitable for a simultaneous multipleband receiver. Further, the outputs of the radio frequency paths aresummed before entering the mixer 5 at a radio frequency summing node.Summation in current domain is most favorable as the summation isachieved by simply routing the outputs together.

The receiver 40 of FIG. 5 can be further improved by connecting thesummation node to the mixer 5 via a current driver 15 as shown in thereceiver 50 of FIG. 8. Without this current driver, the radio frequencysumming node will have a higher signal level than in the single pathreceiver, thus stressing the linearity requirements of the low noisetransconductance 14, and adding more low noise transconductances inparallel to increase the number of parallel inputs increases thecapacitive load of the summing node, thus reducing the bandwidth of thefront-end. Further, the mixer linearity is reduced when feeding it froma finite impedance circuit, here caused by parasitic capacitance inrouting network of the summation node. Therefore, the number of radiofrequency paths in parallel will be limited.

An example of the implementation of the current driver 15 is illustratedin FIG. 9. The signal from the summing node of the low noisetransconductances 14 is coupled via an amplification stage 22 to thegate terminal of a MOSFET transistor 24, which provides signalamplification of the signal. The drain terminal of transistor 24 isconnected to the input of the amplification stage 22 as a feedbacksignal and to the source terminal of a further transistor 25 thatprovides the amplified signal to the input of the mixer 5.

With the current driver 15 it is possible to lower the impedance at thesummation node, making the solution intrinsically wider band andallowing for an increased number of radio frequency branches. The lowimpedance summation node also reduces the voltage swing due to themultiple signals, not degrading the overall linearity. To keep thefrequency response and the linearity performance constant, the impedanceof the current driver 15 should be increased or decreased dynamicallywhen the number of active low noise transconductances 14 decreases orincreases, thus solving the problem of the frequency and linearityperformance dependence on the number of RF inputs enabled. Thepossibility to modify the impedance of the current driver 15 is crucialto keep the performance constant and allow a high number of radiofrequency inputs “transparently”.

The current driver 15 also provides isolation between the radiofrequency summing node and the radio frequency input of the mixer,thereby keeping the loading impedance driving the mixer independent ofthe number of input stages. The capacitive load will not increase at theinput of the passive mixer. This optimizes the mixer performance,especially in terms of linearity.

These advantages, i.e. low impedance summing node, high drivingimpedance for the mixer and isolation between radio frequency inputs andoutput, are achieved in the current driver 15 shown in FIG. 9. If thenumber of low noise transconductances 14 that are active at the sametime increases, the input impedance modification of this stage needed tokeep the performance constant can be done by either making theamplification A of the amplifier 22 variable or changing the biascurrent flowing in transistor 24.

As illustrated in FIG. 9, the current driver 15 may, in someembodiments, also comprise a second stage with the transistors 26 and27. This stage provides a replica of the current signal for I/Qimbalance correction with relatively good isolation from the path withtransistors 24 and 25 and independently of the number of radio frequencyinputs. In some embodiments, the implementation of such a correction isdone through an auxiliary receiver. If the current driver 15 had beenconstant, even the I/Q measurement would have been effected by thenumber of radio frequency inputs.

The stage for I/Q measurement can be used not just for I/Q imbalancecorrection, but also for e.g. power detection, 1/f noise removal andtransmitter signal cancellation. Even in those cases, without anadditional stage keeping constant the overall performance, thosefunctionalities would have been dependent on the number of radiofrequency inputs.

Thus, with an available current replica for a parallel measurement path,providing isolation between the receive path and the I/Q measurementpath, the current driver 15 introduces a new common radio frequency nodesuitable for I/Q imbalance estimation and compensation. This means thatthe receiver 50 implements the solution of receiving multiple bandssimultaneously, without increasing the I/Q imbalance compared to asingle receiver solution, but making available a new radio frequencynode for the I/Q imbalance correction itself.

Thus, the receiver 50 implements the combination of multiple radiofrequency signal bands simultaneously to be combined into a widebandsignal. This solution is implemented with a current summation at radiofrequency to yield only a single source of I/Q imbalance across multiplebands. Moreover, this solution is less sensitive to the parasiticcapacitance due to the multiple radio frequency paths or othercalibration/measurement circuits, allowing a high number of radiofrequency inputs. The addition of a common current stage indeed providesisolation between the radio frequency summing node and the input of themixer and between the main radio frequency signal path and the auxiliarypath used for calibration/measurements.

The apparatus proposed implements a solution of receiving simultaneouslymultiple radio frequency bands to be combined into a wideband signalwithout introducing any additional source of I/Q imbalance compared to asingle band receiver. Moreover, with a single common radio frequencynode available, it is possible to correct the I/Q imbalance itself andalso to implement other signal processing operations like powerdetection, 1/f noise removal and transmitter signal cancellation. Thecurrent topology choice uses a current driver 15 at the summing radiofrequency node, allowing increased number of inputs and reducing theeffect of the capacitive load in the summing node (for example due tothe summing network and the I/Q calibration circuit) without affectingthe linearity and band of the receiver or the performance of the passivemixer.

Among the advantages of this solution with signal combination at radiofrequency, it can be mentioned that it provides a simple and robustdistribution of local oscillator signal, resulting in low powerconsumption. There is one single source of I/Q imbalance due to thesingle quadrature mixer. A single common radio frequency node isavailable to perform I/Q-imbalance correction and also for measurementand/or injection needed for power detection, 1/f noise removal andtransmitter signal cancellation. The implementation of the summing nodein current allows increasing the number of parallel paths, introducing asumming stage with low input impedance proportional to the number ofparallel channels. This property can be improved by technologydownscaling.

FIG. 10 shows an example of a wireless communications system, in which areceiver as described above can be used. Radio frequency signals aretransmitted between two wireless communications devices, which are hereexemplified by a base station 61 and a wireless terminal 62, such as amobile phone, a machine-type communication (MTC) device, or a computer,a tablet device, or other device equipped with a cellular data modem. Inthe base station 61, an antenna 63 is connected to a transmitter part 64and a receiver part 65, which are both connected to a signal processingunit 66. As illustrated, the base station 61 comprises a receiver, e.g.the receiver 50 of FIG. 8, for receiving simultaneously multiple radiofrequency bands. Similarly, in the wireless terminal 62, an antenna 67is connected to a transmitter part 68 and a receiver part 69, which areboth connected to a signal processing unit 70. As illustrated, thewireless terminal 63 comprises a receiver, e.g. the receiver 50 of FIG.8, for receiving simultaneously multiple radio frequency bands.

FIG. 11 shows a flow chart 100 illustrating a method of receiving aplurality of radio frequency bands in a radio frequency receivercomprising a plurality of parallel receiving paths. In step 101, a radiofrequency signal in one of a plurality of radio frequency bands isreceived in each receiving path of the receiver. In each receiving paththe signal is compensated in step 102 for variations in signal strengthof the received radio frequency signal in an automatic gain controlcircuit independently of signal strengths of the signals received byother receiving channels of the radio frequency receiver, and in step103, the compensated radio frequency signal is amplified. In step 104,the amplified and compensated signal is then provided as a currentsignal by a low noise transconductance circuit to a common summationnode, in which the radio frequency signals from the plurality ofparallel receiving paths are combined in step 105 to one combined radiofrequency signal. The combined radio frequency signal is thendown-converted in step 106 to a baseband signal or an intermediatefrequency signal in a mixer circuit.

In other words, there is disclosed a radio frequency receiver 40; 50configured to receive a plurality of radio frequency bands, the radiofrequency receiver comprising a plurality of parallel receiving paths31.1, 31.2, 31.n, wherein each receiving path 31.1; 31.2; 31.n isconfigured to receive a radio frequency signal in one of said pluralityof radio frequency bands and to amplify the received radio frequencysignal in a low noise amplifier, the receiver being configured tocombine amplified radio frequency signals from the plurality of parallelreceiving paths 31.1, 31.2, 31.n to one combined radio frequency signalin a common summation node and to down-convert said combined radiofrequency signal to a lower frequency signal in a mixer circuit 5. Theobject is achieved when each one of said low noise amplifiers comprisesa low noise transconductance circuit 14 configured to provide a currentsignal to drive said common summation node, and an automatic gaincontrol circuit 16 configured to compensate for variations in signalstrength of the radio frequency signal received by the receiving path31.1; 31.2; 31.n comprising the low noise amplifier independently ofsignal strengths of radio frequency signals received by other receivingpaths of the radio frequency receiver.

When the receiver is configured to perform an automatic gain controlfunction separately in each receiver path, simultaneous multiple bandreception can be provided, also where received signal strength variesconsiderably between the received frequency bands.

In some embodiments, the radio frequency receiver further comprises acurrent driver circuit 15 having an input connected to said commonsummation node and an output connected to an input of said mixer circuit5. In this way, it is possible to keep the impedance at the summationnode low, making the solution intrinsically wider band and allowing foran increased number of radio frequency branches. The low impedancesummation node also reduces the voltage swing due to the multiplesignals, so that the overall linearity is not degraded. The currentdriver 15 also provides isolation between the radio frequency summingnode and the radio frequency input of the mixer, thereby keeping theloading impedance driving the mixer independent of the number of inputstages.

The current driver circuit 15 may comprise circuitry for adjusting itsinput impedance in dependence of a number of parallel receiving pathsreceiving radio frequency signals at a given time. The possibility ofincreasing or decreasing the impedance of the current driver 15dynamically when the number of active receiving paths decreases orincreases allows the frequency response and the linearity performance tobe kept constant, so that the frequency and linearity performance doesnot depend on the number of radio frequency inputs enabled.

The current driver circuit 15 may further comprise an auxiliary outputconnected to circuitry for measurement and correction of I/Q imbalance,said auxiliary output being isolated from the output connected to aninput of said mixer circuit. Having a separate I/Q measurement outputisolated from the receive path ensures that I/Q imbalance estimation andcompensation can be performed independently of the number of activeradio frequency bands and without affecting the receive path.

The radio frequency receiver may further be configured to utilize saidauxiliary output for at least one of power detection, 1/f noise removaland transmission signal cancellation.

In some embodiments, the mixer circuit 5 is configured to down-convertsaid combined radio frequency signal to a baseband signal.

A wireless communications device may comprise at least one radiofrequency receiver as described above. In this way, the wirelesscommunications device benefits from the described advantages of theradio frequency receiver. In one embodiment, the wireless communicationsdevice is a base station for a wireless communications system. Inanother embodiment, the wireless communications device is a mobile phoneuse in a wireless communications system.

As mentioned, the invention further relates to a method of receiving aplurality of radio frequency bands in a radio frequency receivercomprising a plurality of parallel receiving paths 31.1, 31.2, 31.n, themethod comprising the steps of receiving 101 in each receiving path aradio frequency signal in one of said plurality of radio frequencybands; compensating 102, in an automatic gain control circuit in eachone of said low noise amplifiers, for variations in signal strength ofthe radio frequency signal received by the receiving path comprising thelow noise amplifier independently of signal strengths of radio frequencysignals received by other receiving paths of the radio frequencyreceiver; amplifying 103 in each receiving path the compensated radiofrequency signal in a low noise amplifier; providing 104 by a low noisetransconductance circuit in each one of said low noise amplifiers acurrent signal to drive a common summation node; combining 105 amplifiedand compensated radio frequency current signals from the plurality ofparallel receiving paths to one combined radio frequency signal in saidcommon summation node; and down-converting 106 said combined radiofrequency signal to a lower frequency signal in a mixer circuit.

When variations in signal strength is compensated by performing anautomatic gain control function separately in each receiver path,simultaneous multiple band reception can be provided, also wherereceived signal strength varies considerably between the receivedfrequency bands.

In some embodiments, the method further comprises the step of providingthe combined radio frequency signal from said common summation node toan input of said mixer circuit by a current driver circuit 15 having aninput connected to said common summation node and an output connected toan input of said mixer circuit. In this way, it is possible to keep theimpedance at the summation node low, making the solution intrinsicallywider band and allowing for an increased number of radio frequencybranches. The low impedance summation node also reduces the voltageswing due to the multiple signals, so that the overall linearity is notdegraded. The current driver 15 also provides isolation between theradio frequency summing node and the radio frequency input of the mixer,thereby keeping the loading impedance driving the mixer independent ofthe number of input stages.

The method may further comprise the step of adjusting an input impedanceof the current driver circuit 15 in dependence of a number of parallelreceiving paths receiving radio frequency signals at a given time. Thepossibility of increasing or decreasing the impedance of the currentdriver 15 dynamically when the number of active receiving pathsdecreases or increases allows the frequency response and the linearityperformance to be kept constant, so that the frequency and linearityperformance does not depend on the number of radio frequency inputsenabled.

The method may further comprise the step of providing by the currentdriver circuit 15 an auxiliary output, isolated from the outputconnected to an input of said mixer circuit, to circuitry formeasurement and correction of I/Q imbalance. Having a separate I/Qmeasurement output isolated from the receive path ensures that I/Qimbalance estimation and compensation can be performed independently ofthe number of active radio frequency bands and without affecting thereceive path.

The method may further comprise the step of further utilizing saidauxiliary output for at least one of power detection, 1/f noise removaland transmission signal cancellation.

In some embodiments, the method may further comprise the step ofdown-converting said combined radio frequency signal in said mixercircuit 5 to a baseband signal.

Although various embodiments of the present invention have beendescribed and shown, the invention is not restricted thereto, but mayalso be embodied in other ways within the scope of the subject-matterdefined in the following claims.

1. A radio frequency receiver configured to receive a plurality of radiofrequency bands, the radio frequency receiver comprising a plurality ofparallel receiving paths, each receiving path being configured toreceive a radio frequency signal in one of the plurality of radiofrequency bands and to amplify the received radio frequency signal in alow noise amplifier, the receiver being configured to combine amplifiedradio frequency signals from the plurality of parallel receiving pathsto one combined radio frequency signal in a common summation node and todown-convert the combined radio frequency signal to a lower frequencysignal in a mixer circuit, each one of the low noise amplifierscomprising: a low noise transconductance circuit configured to provide acurrent signal to drive the common summation node, and an automatic gaincontrol circuit configured to compensate for variations in signalstrength of the radio frequency signal received by the receiving pathcomprising the low noise amplifier independently of signal strengths ofradio frequency signals received by other receiving paths of the radiofrequency receiver.
 2. The radio frequency receiver according to claim1, wherein the radio frequency receiver further comprises a currentdriver circuit having an input connected to the common summation nodeand an output connected to an input of the mixer circuit.
 3. The radiofrequency receiver according to claim 2, wherein the current drivercircuit comprises circuitry for adjusting an input impedance of thedriver circuit in dependence of a number of parallel receiving pathsreceiving radio frequency signals at a given time.
 4. The radiofrequency receiver according to claim 2, wherein the current drivercircuit further comprises an auxiliary output connected to circuitry formeasurement and correction of I/Q imbalance, the auxiliary output beingisolated from the output connected to an input of the mixer circuit. 5.The radio frequency receiver according to claim 4, wherein the radiofrequency receiver is further configured to utilize the auxiliary outputfor at least one of power detection, 1/f noise removal and transmissionsignal cancellation.
 6. The radio frequency receiver according to claim1, wherein the mixer circuit is configured to down-convert the combinedradio frequency signal to a baseband signal.
 7. A wirelesscommunications device comprising at least one radio frequency receiver,the at least one radio frequency receiver configured to receive aplurality of radio frequency bands, the radio frequency receivercomprising a plurality of parallel receiving paths, each receiving pathbeing configured to receive a radio frequency signal in one of theplurality of radio frequency bands and to amplify the received radiofrequency signal in a low noise amplifier, the receiver being configuredto combine amplified radio frequency signals from the plurality ofparallel receiving paths to one combined radio frequency signal in acommon summation node and to down-convert said combined radio frequencysignal to a lower frequency signal in a mixer circuit, each one of thelow noise amplifiers comprising: a low noise transconductance circuitconfigured to provide a current signal to drive said common summationnode, and an automatic gain control circuit configured to compensate forvariations in signal strength of the radio frequency signal received bythe receiving path comprising the low noise amplifier independently ofsignal strengths of radio frequency signals received by other receivingpaths of the radio frequency receiver.
 8. The wireless communicationsdevice according to claim 7, wherein the wireless communications deviceis a base station for a wireless communications system.
 9. The wirelesscommunications device according to claim 7, wherein the wirelesscommunications device is a mobile phone for use in a wirelesscommunications system.
 10. A method of receiving a plurality of radiofrequency bands in a radio frequency receiver comprising a plurality ofparallel receiving paths, the method comprising the steps of: receivingin each receiving path a radio frequency signal in one of the pluralityof radio frequency bands; compensating, in an automatic gain controlcircuit in each one of the low noise amplifiers, for variations insignal strength of the radio frequency signal received by the receivingpath comprising the low noise amplifier independently of signalstrengths of radio frequency signals received by other receiving pathsof the radio frequency receiver; amplifying in each receiving path thecompensated radio frequency signal in a low noise amplifier; providingby a low noise transconductance circuit in each one of the low noiseamplifiers a current signal to drive a common summation node; combiningamplified and compensated radio frequency current signals from theplurality of parallel receiving paths to one combined radio frequencysignal in the common summation node; and down-converting the combinedradio frequency signal to a lower frequency signal in a mixer circuit.11. The method according to claim 10, further comprising the step ofproviding the combined radio frequency signal from the common summationnode to an input of the mixer circuit by a current driver circuit havingan input connected to the common summation node and an output connectedto an input of the mixer circuit.
 12. The method according to claim 11,further comprising the step of adjusting an input impedance of thecurrent driver circuit in dependence of a number of parallel receivingpaths receiving radio frequency signals at a given time.
 13. The methodaccording to claim 11, further comprising the step of providing by thecurrent driver circuit an auxiliary output, isolated from the outputconnected to an input of the mixer circuit, to circuitry for measurementand correction of I/Q imbalance.
 14. The method according to claim 13,further comprising the step of further utilizing the auxiliary outputfor at least one of power detection, 1/f noise removal and transmissionsignal cancellation.
 15. The method according to claim 10, furthercomprising the step of down-converting the combined radio frequencysignal in the mixer circuit to a baseband signal.
 16. The radiofrequency receiver according to claim 2, wherein the mixer circuit isconfigured to down-convert the combined radio frequency signal to abaseband signal.
 17. The radio frequency receiver according to claim 3,wherein the mixer circuit is configured to down-convert the combinedradio frequency signal to a baseband signal.
 18. The radio frequencyreceiver according to claim 5, wherein the mixer circuit is configuredto down-convert the combined radio frequency signal to a basebandsignal.
 19. The method according to claim 11, further comprising thestep of down-converting the combined radio frequency signal in the mixercircuit to a baseband signal.
 20. The method according to claim 14,further comprising the step of down-converting the combined radiofrequency signal in the mixer circuit to a baseband signal.